1. Field of the Invention
The present invention relates to segment type color liquid crystal display devices which display prescribed characters, graphics, etc., in color.
2. Description of the Prior Art
FIG. 1 is a plane view of a segment type liquid crystal display device 230 of the prior art. A plurality of segment electrodes 233 is arranged in the liquid crystal display device in a figure eight. Area Al has a color filter, colored red for example, area A2 has a green color filter and areas A3 to A5 have blue color filters.
FIG. 2 is a cross section of the liquid crystal display device 230. The liquid crystal layer 237 is sandwiched between the glass substrates 231a and 231b transmissive to light and sealed by the sealant resin 238. A plurality of segment electrodes 233 is formed on the surface of the glass substrate 231a toward the liquid crystal layer 237, and also formed on this surface is an orientation film 234a. A plurality of common electrodes 232 is formed on the surface of the glass substrate 231b toward the liquid crystal layer 237, and also formed on this surface is an orientation film 234b. Polarizer plates 235a and 235b are provided on the surfaces of the glass substrates 231a and 231b opposite the liquid crystal layer 237. The red, green and blue color filters 236 mentioned above are formed on areas of the polarizer plates 235a including the segment electrodes 233 by a printing process.
FIG. 3 is a cross section of another configuration of the liquid crystal display device 230. Here, the color filters 236 are formed above the common electrodes 232 by an electrodeposition process or a printing process in order to eliminate color shifts due to parallax in the display area.
In this kind of liquid crystal display device 230, a voltage is applied between the common electrode 232 and the electrodes corresponding to the display areas to be displayed from among the segment electrodes 233 in the area A1, whereby light is allowed to pass through the liquid crystal layer corresponding to the segment electrodes on which the voltage is applied and a red color is displayed. In the other areas, as well, the same method is used to display green and blue.
In the liquid crystal display device disclosed in U.S. Pat. No. 3,840,695, color filters are disposed on the front and rear surfaces of the liquid crystal display element so only one color can be displayed in a single unit area.
Generally, liquid crystals are dependent on the light wavelength, so the blockage of light in the liquid crystal display device is not complete; i.e., light cannot be blocked over the entire visible wavelength band. Therefore when performing positive display (black display color on a white background) in the liquid crystal display device 230, light passes through the liquid crystal layer 237 even when voltage is not applied on the liquid crystal layer 237, so the area Al colored red in FIG. 1 can be distinguished from the other areas, thus degrading the display quality of the liquid crystal display device 230. Also, in the case of negative display (white display color on a black background), the blockage of light by the liquid crystal layer 237 is not complete, so when voltage is not being applied, the shape of the color filters 236 can be seen, thus degrading the display quality.
Further, since the display colors of the display areas are determined by the colors of the color filters at the process of production of the liquid crystal display device, the same display area cannot display different colors, thus detracting from the diversity of display and applicability.
Liquid crystal display devices utilizing fine stripe-shaped color filters were disclosed in Japanese Laid-Open Patent Publications 60-260921, 61-239220 and 62-091917 to solve the above problems.
FIG. 4 is a cross section of a segment type color liquid crystal display device 301 utilizing stripe-shaped color filters. A color filter 305 is provided on nearly the entire surface of one surface of the glass substrate 304a, which is transmissive to light, by means of a printing process or an electrodeposition color process, and a plurality of segment electrodes 306 is formed on the color filter 30 corresponding to predetermined display areas. The color filter 305 is formed in a stripe shape parallel to the surface of the paper in FIG. 4 and a plurality of red, green and blue filters is arranged in a predetermined order in a direction perpendicular to the surface of the paper in FIG. 4. The segment electrodes 306 comprise an electrode for red, an electrode for green and an electrode for blue corresponding to the red filter, green filter and blue filter.
On one surface of the glass substrate 304b transmissive to light is formed a plurality of common electrodes 308 over the areas containing the display areas corresponding to the segment electrodes 306, and then an orientation film 307b is formed on the surface. The glass substrates 304a and 304b are disposed so that the surfaces face each other on which the orientation films 307a and 307b are formed, and a twisted nematic liquid crystal layer 309 is sandwiched between the glass substrates 304a and 304b and sealed with a sealant resin 310. The surfaces of the glass substrates 304a and 304b opposite the liquid crystal layer 309 are each provided with polarizer plates 311 and 312.
Here, the polarizer plates 311 and 312 are disposed to achieve parallel polarization, and so-called "normally black" display is performed; i.e., light is not allowed to pass through the liquid crystal display when voltage is not applied. By applying a voltage on one of the three electrodes which make up the segment electrodes 306, the desired color can be displayed at the display area corresponding to the segment electrodes 306. For example, when a voltage is applied on only the electrode for red, red is displayed in the display area, and when voltages are simultaneously applied on the electrode for red and the electrode for green, yellow can be displayed by the mixing of the red and green colors. Further, when voltages are simultaneously applied on the electrode for red, the electrode for green and the electrode for blue, white can be displayed.
The color liquid crystal display device 301 described above is provided with transparent electrodes corresponding to each of the three color filters; e.g., red filter, green filter and blue filter, formed in the predetermined display area, and by selecting the transparent electrode(s) on which the operating voltage is to be applied, a maximum of eight colors can be displayed.
However, since each of the display colors can only be displayed at one gradation level, there is a limit to the variety of display colors, so the display lacks diversity.
A color liquid crystal display device can be conceived which realizes display of gray scale levels of color (variable density) wherein the light transmissivity of the liquid crystal layer is changed and gray scale display performed by using a drive circuit capable of controlling the amplitude of operating voltage applied or the time of operating voltage is applied on the liquid crystal layer. In this color liquid crystal display device, it is necessary to develop a drive circuit for controlling the amplitude of operating voltage or the time of the operating voltage.
Further, a color liquid crystal display device has been proposed which achieves gray scale display by varying the area on the transparent electrode on which voltage is applied by dividing the transparent electrodes, which are formed so they correspond to the color filters, up into a greater number and driving the divided electrodes independently
FIG. 5 is a cross section showing the configuration of a color liquid crystal display device 101 capable of gray scale display. The color liquid crystal display device 101 comprises a pair of transparent substrates 102a and 102b, and segment electrodes 100 are formed on one side of the transparent substrate 102a in the area corresponding to the shape to be displayed. The segment electrodes 100 are made up of the electrodes 100R, 100G and 100B corresponding to the three color filters 105R, 105G and 105B. The electrodes 100R, 100G and 100B are each made up of the first and second segment electrodes 103R and 104R, 103G and 104G, and 103B and 104B.
The color filter 105 is formed on the transparent substrate 102a on which the segment electrodes 100 are formed. The color filter 105 comprises color filters of three colors; e.g., the red filter 105R, the green filter 105G and the blue filter 105B, formed in a stripe shape and a black light mask layer 105BL disposed between each of the filters, and it is formed over nearly the entire surface of one side of the transparent substrate 102a by a printing process or other process. The orientation film 106 is then formed on the surface of the color filter 105.
The common electrode 107 is formed on one surface of the transparent substrate 102b in an area containing at least the segment electrodes 100, and then an orientation film 106b is formed on that surface.
The transparent substrates 102a and 102b are disposed so that their respective surfaces on which the orientation films 106a and 106b are formed oppose each other, and the liquid crystal layer 108 is sandwiched between the transparent substrates 102a and 102b and sealed by the sealant 109. The polarizer plates 110 and 111 are disposed on the surfaces of the transparent substrates 102a and 102b opposite the liquid crystal layer 108.
When producing the color liquid crystal display device 101, the electrodes corresponding to the fine color filters are further divided, so high precision processing technology is required and productivity is low. Further, the first and second segment electrodes 103 and 104 are formed on the same flat surface; i.e., on the transparent substrate 102a, so it is necessary to leave a fixed interval between adjacent electrodes when they were formed in order to electrically isolate (insulate) them, thus resulting in reduced illumination ratio (illuminated area/display area.times.100(%)). When the interval between the first and second segment electrodes 103 and 104 was formed as narrow as possible in order to improve illumination ratio, short circuits between electrodes and broken wires often occurred, so in practice they were impossible to produce.
The display image of the segment type color liquid crystal display devices 230 and 301 is determined by the shape of the segment electrodes 233 and 306 and cannot be changed subsequent to production of the color liquid crystal display devices 230 and 301, so these devices lack in diversity of display.
In order to enhance the diversity of display, a color liquid crystal display device was proposed in which a liquid crystal element for color display and a liquid crystal element for black and white display were stacked.
FIG. 6 is a cross section of the two-layered liquid crystal display device 401. The liquid crystal display device 401 comprises two liquid crystal elements for display: the liquid crystal element 402 for color display and the liquid crystal element 403 for black and white display.
The liquid crystal element 402 for color display comprises a pair of transparent substrates 404a and 404b. A plurality of segment electrodes 405 transmissive to light are formed on one surface of the transparent substrate 404a, and the color filter 406 is formed on the transparent substrate 404a on which the segment electrodes 405 were formed. The color filter 406 has the same configuration as the color filter 305 in FIG. 4. The orientation film 407a is formed on the surface of the color filter 406.
The light transmissive common electrodes 408 are formed in an area on one surface of the transparent substrate 404b containing at least the segment electrodes 405, and then an orientation film 407b is formed on that surface. The transparent substrates 404a and 404b are disposed so that their respective surfaces on which the orientation films 407a and 407b are formed oppose each other. The liquid crystal layer 409 is sandwiched between the transparent substrates 404a and 404b and sealed with the sealant 410.
The liquid crystal element 403 for black and white display comprises a pair of transparent substrates 411a and 411b, a plurality of segment electrodes 412 formed on one surface of the transparent substrate 411a in a range corresponding to the predetermined shape to be displayed in the display area, and an orientation film 413a formed on the transparent substrate 411a on which the segment electrodes 412 are formed. The common electrodes 414 transmissive to light are formed in an area on one surface of the transparent substrate 411b containing at least the segment electrodes 412, and then the orientation film 413b is formed on the transparent substrate 411b on which the common electrodes 414 were formed. The transparent substrates 411a and 411b are disposed so that their respective surfaces on which the orientation films 413a and 413b are formed oppose each other. The liquid crystal layer 415 is sandwiched between the transparent substrates 411a and 411b and sealed with the sealant 416.
The liquid crystal element 402 for color display and the liquid crystal element 403 for black and white display are stacked together so that the transparent substrate 404a and the transparent substrate 411b face each other. The polarizer plate 417 is disposed on the surface of the transparent substrate 404b opposite the liquid crystal layer 409, and the polarizer plate 418 is disposed on the surface of the transparent substrate 411a opposite the liquid crystal layer 415. These polarizer plates 417 and 418 are also disposed so that they are parallel polarization.
The angle of twist of the molecules of both the liquid crystal layer 409 of the liquid crystal element 402 for color display and the liquid crystal layer 415 of the liquid crystal element 403 for black and white display is 90.degree.. Also, the molecules of the liquid crystal layer 409 have a levorotatory orientation, while the molecules of the liquid crystal layer 415 have a dextrorotatory orientation. That is, the directions of spiral of the molecules in the two liquid crystal elements 402 and 403 are set so they are opposite each other. Further, the direction of orientation of the liquid crystal molecules nearest the transparent substrate 404a in the liquid crystal layer 409 and the direction of orientation of the liquid crystal molecules nearest the transparent substrate 411b in the liquid crystal layer 415 are set so they are perpendicular to each other.
FIG. 7 is a plane view of the liquid crystal display device 401. The display area A6 is the area formed by the segment electrodes 405 of the liquid crystal element 402 for color display and it is capable of color display. The display area A7 is the area formed by the segment electrodes 412 of the liquid crystal element 403 for black and white display, and it is capable of black and white display. Here, the polarizer plates 417 and 418 are disposed so they are parallel polarization. Therefore, when voltage is not applied on the liquid crystal layer, the incident light passes through the liquid crystal layer, thus facilitating so-called "normally white display" in which the background color is white.
TABLE 1 shows the relationship between the operating condition of the liquid crystal element 402 for color display and the liquid crystal element 403 for black and white display in each of the display areas in the liquid crystal display device 401, the direction of polarization of the incident light on and the light passing through the liquid crystal element 402 for color display and the liquid crystal element 403 for black and white display, and the color displayed in the display area. Here, the direction of the polarizer plates 417, 418 is parallel to the surface of the paper in FIG. 6.
TABLE 1 ______________________________________ Ele- ment Element for Dis- for Trans- black & Trans- Trans- play color mitted white mitted mitted/ area display light display light blocked Color ______________________________________ A6 R OFF Perpen- OFF Parallel Trans- Red dicular mitted G ON Parallel Perpen- Blocked dicular B ON Parallel Perpen- Blocked dicular A7 R OFF Perpen- ON Perpen- Blocked Black dicular dicular G OFF Perpen- Perpen- Blocked dicular dicular B OFF Perpen- Perpen- Blocked dicular dicular A8 R OFF Perpen- ON Perpen- Blocked Light dicular dicular blue G ON Parallel Parallel Trans- mitted B ON Parallel Parallel Trans- mitted Resi- R OFF Perpen- OFF Parallel Trans- White dual dicular mitted G OFF Perpen- Parallel Trans- dicular mitted B OFF Perpen- Parallel Trans- dicular mitted ______________________________________
As shown in TABLE 1 above, in the case in which red is displayed in the display area A6 and black is displayed in the display area A7, light blue is displayed in the display area A8 where the display area A6 and the display area A7 overlap.